The transmission level of signals, in terms of transmitter wattage output, is often required to remain within specified tolerances. For example, the output power transmitted by cellular radio telephones and mobile radios is governed by Federal Regulations. If insufficient RF power is produced to transmit a modulated carrier, the receiver cannot accurately demodulate the voice signals and decode the data. If a transmitter transmits the carrier signal with too much power, the performance of adjacent channels may be affected and possible jamming thereof may result.
In many wireless communication systems, the receiver often includes circuits for measuring the signal strength of received signals. A coded signal is then transmitted by the receiver to a base station to indicate the signal strength of the carrier received by the receiver. The base station then dispatches a transmission to the transmitter to increase or decrease the power by which the modulated carrier is transmitted. Once the power level is set, it is maintained at such level, until signaled by the base station to again change the transmitter power level. In practice, the power level of a transmitter may change frequently, due to distance, obstructions, etc., between the transmitter and receiver. Once the power level of a transmitter is established by the base station, or otherwise, the transmitter circuits sense the power level and maintain such level within predefined limits.
The measurement of power output by a transmitter can be accomplished in many different ways. One technique for sensing the transmitted power is to sense the peak voltage of the transmitter carrier and utilize the peak voltage magnitude for comparison with a reference. Although this technique provides an accurate representation of the transmitted power, the associated circuitry is complex, costly and often component intensive.
The most common technique for measuring transmitter power is to sample a portion of the power produced by the transmitter and convert the sampled signal to a DC voltage. The DC voltage is then compared with a reference level by the use of a comparator, or the like. An error signal is generated and fed back to the transmitter to control the transmitter power accordingly.
The sampling of the transmitted power can be efficiently accomplished by the use of a directional coupler. With such a device, the forward transmitted power is sampled and provided as an output at a "forward" port of the device. In the event reflected power is of interest, the reflected power of a transmitter is sampled and provided as an output at a "reverse" port of the directional coupler. Again, the sampled power at the forward port is converted to a DC voltage and utilized in a feedback loop to increase or decrease the transmitter power to maintain the same at a specified level.
In any electrical circuit involving a semiconductor junction, such as in a diode or transistor, changes in the ambient temperature change the operating characteristics of the semiconductor device, thereby modifying the operating conditions. A change in the operating conditions due to temperature represents an error or inaccuracy. In diode rectifying circuits conventionally utilized in sensing transmitter power, a change in the temperature of the diode causes a change in the rectified voltage and the feedback error signal, despite the fact that the transmitter power remains constant.
To circumvent this inaccuracy due to temperature, various types of temperature compensation circuits have been devised. In U.S. Pat. No. 4,523,155 by Walczak et al, a temperature compensation circuit is connected in parallel with the rectifier circuit. With this arrangement, as the temperature attempts to change the operating characteristics of the rectifier circuit, the temperature compensation circuit provides an offsetting voltage to remove the effects of temperature changes. The disadvantage of such an arrangement is that when the temperature compensation circuit is connected in parallel with the rectifier circuit, the sensitivity to low power levels is compromised. As a result, the rectifier circuit becomes less sensitive to power level changes, especially when the transmitter operates at low power levels.
U.S. Pat. No. 5,113,336 by Takahashi et al. discloses another type of a temperature compensated level detector utilizing a directional coupler or transmission line to sample the RF power. Again, the temperature compensation circuit is shunted across the level detector that rectifies the RF current, thereby reducing the sensitivity of the level detector.
From the foregoing, it can be seen that a need exists for an improved technique in sensing power levels and providing temperature compensation without compromising the sensitivity of the circuits. Another need exists for a temperature compensated power detector that is cost effective, has few components and accurately removes the effects of changes in temperature on semiconductor components.